Although it’s possible to set a negative revenue the best option is to probably develop and use a self-describing JSON schema.

Iglu Central has some ecommerce related schemas that you could likely use as a starting point for a refund schema. You can then use SelfDescribingJson (or the equivalent in another tracker) to send this data to the collector. This allows a great deal of flexibility in sending metadata such as refund amount, product(s) etc.